EP4250315A1

EP4250315A1 - Safety system and safety control method for preventing molten corium from melting through rpv

Info

Publication number: EP4250315A1
Application number: EP20965463.1A
Authority: EP
Inventors: Dekui ZHAN; Shaoxiong XIA; Peng Chen; Hui Fu; Zijie WU; Dongyu HE; Chunrong LIU
Original assignee: China General Nuclear Power Corp; China Nuclear Power Technology Research Institute Co Ltd; CGN Power Co Ltd
Current assignee: China General Nuclear Power Corp; China Nuclear Power Technology Research Institute Co Ltd; CGN Power Co Ltd
Priority date: 2020-12-16
Filing date: 2020-12-16
Publication date: 2023-09-27
Also published as: EP4250315A4; WO2022126445A1; CN116368580A

Abstract

Disclosed are a safety system and safety control method for preventing a molten corium from melting through an RPV The safety system comprises a pit for accommodating a reactor pressure vessel (RPV), a cooling water tank, a cooling channel and a water injection line; the cooling water tank is arranged around the pit, and the cooling channel is arranged at the bottom of the pit and is in communication with the cooling water tank; the water injection line is connected between the cooling water tank and the pit, and cooling water in the cooling water tank is injected, under the action of gravity, into the pit in a passive manner by means of the water injection line. By means of the safety system for preventing the molten corium from melting through the RPV of the present invention, under working conditions including severe accidents, the passive water injection into the pit by using gravitational potential energy can be achieved without relying on an external alternating current power supply, and the system is simple and has high reliability.

Description

FIELD OF THE INVENTION

The invention relates to the technical field of nuclear power, in particular to a safety system and a safety control method for preventing molten corium from melting through an RPV

DESCRIPTION OF THE RELATED ART

For low-power nuclear power plants (200-300MWe), the core will undergo the process of water loss, temperature rise and meltdown in the event of a severe accident, and the molten core will collapse and fall to a lower head of a reactor pressure vessel (RPV). If the high-temperature molten corium cannot be cooled effectively in time, the molten corium may melt through the RPV, resulting in a large amount of radioactive material leakage. In order to prevent the RPV from being melted through under severe accident conditions, in view of the low thermal power characteristics of the low-power nuclear power plants, an in-vessel corium retention (IVR) strategy has been adopted in which a water injection system is set up in a reactor pit. Different from a natural circulation cooling method used by high-power nuclear power plants for RPV cooling, the low-power nuclear power plants adopt a tank boiling method for RPV cooling, which takes away the sensible heat and decay heat of the molten corium and finally makes the molten corium cool down and stay in the lower head of the reactor pressure vessel, thus ensuring the integrity of the pressure vessel.
Most of the existing large-scale pressurized water reactor nuclear power plants (with a thermal power above 600MWeu) adopt the corium cooling and retention strategy to achieve the in-vessel corium cooling and retention under severe accidents, thereby maintaining the integrity of the RPV under severe accidents. Generally speaking, large nuclear power plants set up a thermal insulation layer flow channel inside the reactor pit. In the event of a severe accident accident, water is injected into the reactor pit by means of passive water injection or active water injection; after the reactor pit is submerged, the cooling water at the outer wall surface of the RPV is driven by means of natural circulation or forced circulation to conduct convective heat transfer. This design involves the design of a metal insulation layer, and a water inlet, an exhaust hole and the circulation flow channel of the insulation layer. However, this structure can be greatly affected by the structural integrity of the insulation layer, and reliability of the water inlet of the insulating layer and an exhaust hole cover plate can also affect the circulating flow. In addition, the installation cost, manufacturing cost and maintenance cost of the above devices are relatively high.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a simple and reliable safety system for preventing molten corium from melting through an RPV and a safety control method for preventing molten corium from melting through an RPV
The technical solution adopted by the present invention to solve the technical problem is providing a safety system for preventing molten corium from melting through an RPV, which includes a reactor pit for receiving the RPV, a cooling water tank, a cooling channel and a water injection pipeline.
The cooling water tank is arranged at the outer periphery of the reactor pit, the cooling channel is arranged at the bottom of the reactor pit and in communication with the cooling water tank; the water injection pipeline is connected between the cooling water tank and the reactor pit, cooling water in the cooling water tank is passively injected into the reactor pit through the water injection pipeline under the action of gravity.
Preferably, a shielding sealing layer is provided at the pit opening of the reactor pit, which seals the pit opening while leaving a steam channel; the steam channel is located between the shielding sealing layer and the outer wall surface of the RPV
Preferably, a specular reflection layer is provided on the inner peripheral wall surface and the inner bottom surface of the reactor pit.
Preferably, the safety system further includes at least one anti-convection plate arranged on the inner peripheral wall surface of the reactor pit.
The first end of the anti-convection plate is connected to the specular reflection layer, and the opposite second end is inclined upward and close to the outer wall surface of the RPV
Preferably, the interval between the second end of the anti-convection plate and the outer wall surface of the RPV is 40mm-60mm.
Preferably, the water injection pipeline is provided with a control valve for controlling on-off of the water injection pipeline.
Preferably, the reactor pit is made of stainless steel plates, and the side wall of the reactor pit forms the inner side wall of the cooling water tank.
Preferably, the bottom plate of the reactor pit includes two stainless steel plates that are spaced vertically, and the interlayer between the two stainless steel plates forms the cooling channel.
The present invention further provides a safety control method for preventing molten corium from melting through an RPV, which adopts the safety system for preventing molten corium from melting through the RPV according to any one of above, and the safety control method includes the following steps: in the event of a severe accident, before the molten corium collapses to the lower head of the RPV, the cooling water in the cooling water tank is passively injected into the reactor pit through the water injection pipeline under the action of gravity.
Preferably, the cooling water in the reactor pit boils and evaporates after exchanging heat with the RPV, which takes away the heat on the outer surface of the RPV, and at the same time transfers the heat to the cooling water tank through thermal convection and thermal conduction
Preferably, the safety control method further includes the following steps: under extreme conditions in which the RPV is melted through by the molten corium, the cooling water in the cooling channel at the bottom of the reactor pit exchanges heat with the cooling water that enters the reactor pit and is located below the RPV, so as to cool the bottom of the reactor pressure vessel to take away the heat of the molten corium and prevent the molten corium from burning through the bottom plate of the reactor pit.
Preferably, part of the water vapor in the reactor pit is discharged outward through the steam channel at the pit opening; the discharged water vapor is condensed into water and then returned into the cooling water tank.
Preferably, the safety control method further includes the following steps: under normal working conditions, a cooling space between the reactor pit and the RPV forms a heat preservation space formed by relatively still air, the heat of the RPV is transferred to the air in the reactor pit, and then conducted to the cooling water tank through the side wall of the reactor pit.
The present invention has the following beneficial effects: in the event of a severe accident working condition, water can be passively injected into the reactor pit under the action of gravity, without relying on an external AC power supply; the system is simple and has high reliability, which can realize in-vessel corium retention for low-power nuclear power plants, thereby improving the safety of nuclear reactors, and reducing the risk of large-scale release of radioactive materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, in which:

Fig. 1 is a top view of a structure of a safety system for preventing molten corium from melting through an RPV according to an embodiment of the present invention; and
Fig. 2 is a cross-sectional view of a structure of a safety system for preventing molten corium from melting through an RPV according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In order to have a better understanding of the technical features, objectives and results of the present invention, the specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
As shown in Figs. 1 and 2, a safety system for preventing molten corium from melting through an RPV according to an embodiment of the present invention includes a reactor pit 10 for receiving a reactor pressure vessel (RPV) 100, a cooling water tank 20, a cooling channel 30, and a water injection pipeline 40.
The reactor pressure vessel 100 is suspended in the reactor pit 10, with a space left between the outer surface (including an outer wall surface and a bottom surface) of the reactor pressure vessel 100 and the reactor pit 10 to form a cooling space for circulation of injected cooling water or air therein. The cooling water tank 20 is arranged at the outer periphery of the reactor pit 10, which can provide cooling water for the cooling space, and can also perform heat exchange with the cooling water in the reactor pit 10 to realize the cooling of the reactor pressure vessel 100. The cooling channel 30 is arranged at the bottom of the reactor pit 10 and in communication with the cooling water tank 20, such that the cooling channel 30 is also filled with cooling water and hence can perform heat exchange with the cooling water at the bottom of the reactor pit 10, thereby realizing the cooling of the bottom of the reactor pressure vessel 100. The solution of double-layer in-vessel corium retention (IVR) is thus accomplished.
The water injection pipeline 40 is connected between the cooling water tank 20 and the reactor pit 10. The cooling water in the cooling water tank 20 is passively injected into the reactor pit 10 through the water injection pipeline 40 under the action of gravity, without relying on an external AC power supply, which results in high reliability. Preferably, the water injection pipeline 40 is connected between the upper end of the cooling water tank 20 and the upper end of the reactor pit 10, so that the cooling water can be passively injected into the reactor pit 10 and the submerging cooling water level can be above an active section of the core.
The water injection pipeline 40 is provided with a control valve 41 for controlling on-off of the water injection pipeline 40. The control valve may be an electric valve.
As shown in Fig. 2, in this embodiment, the reactor pit 10 is made of stainless steel plates, so that the side wall and the bottom plate of the reactor pit 10 are all formed by stainless steel plates. Dimensions, such as the inner diameter and depth of the reactor pit 10, are set according to the requirements of the reactor pressure vessel 100. The side wall of the reactor pit 10 forms the inner side wall of the cooling water tank 20. Similarly, the outer side wall of the cooling water tank 20 is also made of stainless steel plates. The bottom plate of the reactor pit 10 includes two stainless steel plates that are vertically spaced, and an interlayer between the two stainless steel plates forms a cooling channel 30. Two ends of the cooling channel 30 can be directly opened to communicate with the cooling water tank 20, or each provided with a vertical plate for support the stainless steel plates, and a through hole is defined through each vertical plate to realize the circulation of the cooling water.
The reactor pressure vessel 100 is arranged in the reactor pit 10, occupying a majority of the space in the reactor pit 10. The volume of the cooling space between the reactor pressure vessel 100 and the inner wall surface of the reactor pit 10 is small, so that the cooling space can be filled with cooling water submerging the reactor pit 10 in a short time. After the cooling water enters the reactor pit 10, the reactor pressure vessel 100 is directly cooled by means of the tank boiling method, and the cooling water for the molten corium in the lower head of the reactor pressure vessel 100 is taken away to maintain the integrity of the reactor pressure vessel 100.
In order to reduce the heat dissipation caused by convection of air inside the reactor pit 10, a shielding sealing layer 50 is provided at the pit opening of the reactor pit 10 to block the pit opening while leaving a steam channel 200. The steam channel 200 is located between the shielding sealing layer 50 and the outer wall surface of the reactor pressure vessel. The steam channel 200 communicates the cooling space in the reactor pit 10 with a large space outside the reactor pit 10 (containment large space), for allowing the steam in the reactor pit 10 to be discharged out of the reactor pit 10 through the steam channel 200.
A specular reflection layer 60 is provided on the inner peripheral wall surface (i.e. the inner wall surface of the side wall) and the inner bottom surface (i.e. the upper surface of the bottom plate) of the reactor pit 10 to reduce heat dissipation caused by thermal radiation.
The safety system of the present invention further includes at least one anti-convection plate 70 disposed on the inner peripheral wall surface of the reactor pit 10. The anti-convection plate 70 has opposite first and second ends, the first end of the anti-convection plate 70 is connected to the specular reflection layer 60, and the second end is inclined upward and close to the outer wall surface of the reactor pressure vessel 100, such that the anti-convection plate 70 is arranged within the reactor pit 10 in an upward inclined manner.
The arrangement of the anti-convection plate 70 can further reduce air convection at the outer wall surface of the reactor pressure vessel 100 and, at the same time, it can also ensure steam overflow when the tank boiling occurs at the outer wall surface of the RPV after the reactor pit 10 is submerged with water under severe accident conditions.
When a number of the anti-convection plates 70 are provided, they are distributed in the reactor pit 10 at intervals along a height direction of the reactor pit 10.
In some embodiments, the interval between the second end of the anti-convection plate 70 and the outer wall surface of the reactor pressure vessel 100 is 40mm-60mm.
In an exemplary embodiment, the cooling water tank 20 is a large cooling water tank, surrounding the periphery of the reactor pit 10, the volume of the cooling water tank 20 is greater than 500m³, and the wall thickness of the cooling water tank 20 is 20-40mm. The free volume of the reactor pit 10 (i.e. the volume of the cooling space) is 7m³. After the cooling water in the reactor pit 10 is heated by the RPV, the heat can be directly transferred to the cooling water tank 20. A small amount of steam may be discharged into a containment via the steam channel 200 at the top of the reactor pit 10.
The distance between the reactor pressure vessel 100 and the bottom of the reactor pit 10 is relatively small, which is about 200mm. Under extreme working conditions, assuming that the molten corium melts through the RPV, the corium will reset to the bottom of the RPV At this time, due to the presence of the cooling water tank 20 and the cooling channel 30 at the bottom of the reactor pit 10, a secondary corium cooling and retention device is formed, which finally cools and retains the molten corium inside the reactor pit 10.
The safety control method for preventing the molten corium from melting through the RPV of the present invention is realized by the above-described safety system, and the safety control method may include the following steps: when a severe accident occurs, before the molten corium collapses to the lower head of the reactor pressure vessel 100, the cooling water in the cooling water tank 20 is passively injected into the reactor pit 10 through the water injection pipeline 40 under the action of gravity.
The initial level of the water surface of the cooling water tank 20 is above the upper surface of the active section of the core, and the volume of the cooling water above the upper surface of the active section of the core can satisfy that the submerging water level after the water injection is still above the active section of the core.
The cooling water in the reactor pit 10 boils and evaporates after exchanging heat with the reactor pressure vessel 100, which takes away the heat from the outer surface of the reactor pressure vessel 100, and at the same time transfers the heat to the cooling water tank 20 through thermal convection and thermal conduction. Part (a small amount) of the water vapor in the reactor pit 10 is discharged to the outside through the steam channel 200 at the pit opening; the discharged water vapor is condensed into water and then returned to the cooling water tank 20, so as to provide a long-term stable heat sink for implementation of the in-vessel corium retention (IVR).
Under the extreme working condition that the reactor pressure vessel 100 is melted through by the molten corium, the cooling water in the cooling channel 30 at the bottom of the reactor pit 10 exchanges heat with the cooling water that enters the reactor pit 10 and is located below the reactor pressure vessel 100, so as to cool the bottom of the reactor pressure vessel 100 to take away the heat of the molten corium and prevent the molten corium from burning through the bottom plate of the reactor pit 10.
In addition, in combination with the safety system of the present invention, under normal working conditions (no severe accident occurring), the cooling space between the reactor pit 10 and the reactor pressure vessel 100 forms a heat preservation space formed by relatively still air, the heat of the reactor pressure vessel 100 is transferred to the air in the reactor pit 10, and then conducted to the cooling water tank 20 through the side wall of the reactor pit 10.
The above are only embodiments of the present invention and do not limit the scope of the present invention. Any equivalent structure or equivalent process transformations made based on the content of the description and drawings of the present invention, or direct or indirect application in other related technical fields, fall equally within the patent scope of the present invention.

Claims

A safety system for preventing molten corium from melting through an RPV, characterized in that the safety system comprises a reactor pit for receiving the RPV, a cooling water tank, a cooling channel and a water injection pipeline;
wherein the cooling water tank is arranged at the outer periphery of the reactor pit, the cooling channel is arranged at the bottom of the reactor pit and in communication with the cooling water tank; the water injection pipeline is connected between the cooling water tank and the reactor pit, cooling water in the cooling water tank is passively injected into the reactor pit through the water injection pipeline under the action of gravity.
The safety system for preventing molten corium from melting through the RPV according to claim 1, characterized in that a shielding sealing layer is provided at the pit opening of the reactor pit, which blocks the pit opening while leaving a steam channel; the steam channel is located between the shielding sealing layer and the outer wall surface of the RPV
The safety system for preventing molten corium from melting through the RPV according to claim 1, characterized in that a specular reflection layer is provided on the inner peripheral wall surface and the inner bottom surface of the reactor pit.
The safety system for preventing molten corium from melting through the RPV according to claim 3, characterized in that the safety system further comprises at least one anti-convection plate arranged on the inner peripheral wall surface of the reactor pit;
wherein the first end of the anti-convection plate is connected to the specular reflection layer, and the opposite second end is inclined upward and close to the outer wall surface of the RPV
The safety system for preventing molten corium from melting through the RPV according to claim 4, characterized in that an interval between the second end of the anti-convection plate and the outer wall surface of the RPV is 40mm-60mm.
The safety system for preventing molten corium from melting through the RPV according to claim 1, characterized in that the water injection pipeline is provided with a control valve for controlling on-off of the water injection pipeline.
The safety system for preventing molten corium from melting through the RPV according to any one of claims 1-6, characterized in that the reactor pit is made of stainless steel plates; the side wall of the reactor pit forms the inner side wall of the cooling water tank.
The safety system for preventing molten corium from melting through the RPV according to claim 7, characterized in that a bottom plate of the reactor pit comprises two stainless steel plates that are spaced vertically, and the interlayer between the two stainless steel plates forms the cooling channel.
A safety control method for preventing molten corium from melting through an RPV, characterized in that the safety system for preventing molten corium from melting through the RPV according to any one of claims 1-8 is adopted, and the safety control method comprises the following steps: in the event of a severe accident, before the molten corium collapses to the lower head of the RPV, the cooling water in the cooling water tank is passively injected into the reactor pit through the water injection pipeline under the action of gravity.
The safety control method for preventing molten corium from melting through the RPV according to claim 9, characterized in that the cooling water in the reactor pit boils and evaporates after exchanging heat with the RPV, which takes away the heat on the outer surface of the RPV, and at the same time transfers the heat to the cooling water tank through thermal convection and thermal conduction.
The safety control method for preventing molten corium from melting through the RPV according to claim 10, characterized by further comprising the following steps: under extreme conditions in which the RPV is melted through by the molten corium, the cooling water in the cooling channel at the bottom of the reactor pit exchanges heat with the cooling water that enters the reactor pit and is located below the RPV, so as to cool the bottom of the reactor pressure vessel to take away the heat of the molten corium and prevent the molten corium from burning through the bottom plate of the reactor pit.
The safety control method for preventing molten corium from melting through the RPV according to claim 10, characterized in that part of the water vapor in the reactor pit is discharged outward through the steam channel at the pit opening; the discharged water vapor is condensed into water and then returned into the cooling water tank.
The safety control method for preventing molten corium from melting through the RPV according to any one of claims 9-12, characterized by further comprising the following steps: under normal working conditions, a cooling space between the reactor pit and the RPV forms a heat preservation space formed by relatively still air, the heat of the RPV is transferred to the air in the reactor pit, and then conducted to the cooling water tank through the side wall of the reactor pit.